In the field of audio signal processing generally and in hearing aid applications in particular, a need exists to manufacture miniaturized audio components such as amplifiers, compressors, expanders and filters. Because of the complexity of the functions provided by these devices, it is difficult to manufacture these circuits in a topology which functions reliably at power supply voltages as low as 1 volt. Today, many audio devices are being designed to operate at very low voltages. Operation at low voltages allows these devices to be powered by a single cell battery. However, operation from a single cell battery severely limits the amount of total power available to the circuitry powered therefrom. Therefore, it is desirable to design low voltage circuitry wherein the current consumption and power dissipation of the circuitry are minimized.
Class AB amplifiers are well known devices which are used for low power and low voltage audio applications such as hearing aids. Although other types of amplifiers, such as Class A and Class B amplifiers, have been used for various audio applications, Class AB amplifiers are superior to Class A amplifiers in minimizing power consumption and are superior to Class B amplifiers in minimizing crossover and harmonic distortion. However, the typical drawback of prior art Class AB amplifiers is a tradeoff between reducing quiescent power dissipation and distortion and available output power. Known prior art designs fall short of reducing power dissipation and distortion to a minimum while maintaining the required output power capability.
One prior art amplifier that is designed to achieve both of the competing goals of reducing quiescent power dissipation and maintaining gain within the limitations required of a hearing aid application is employed in the following hearing aid devices manufactured by Resound Corporation: the ED3 series, the BT2 series, and the BTP series. This amplifier is depicted in FIGS. 1 and 2, and has a symmetrical configuration of transistors and other discrete components which are ideally matched to each other. More specifically, both the transconductance (voltage amplification) portion and the driver portion of the topology employ current mirror configurations. This design effectively reduces quiescent current consumption typical of other prior art designs by over 50%.
Although the amplifier shown in FIGS. 1 and 2 improves upon other prior art amplifiers by maintaining gain characteristics of the amplifier while reducing power dissipation, its design still forgoes some improvement in power consumption in achieving its goal. This is so because the quiescent current cannot be independently controlled from the current gain of the circuit. It is therefore desirable to design an amplifier in which the quiescent current can be set independently from the circuit gain. By structurally decoupling the quiescent current from the gain components of the circuit, a low power-high gain amplifier can better accommodate the power and size limitations of hearing aid applications.
It is therefore an object of the present invention to provide an amplifier in which the power consumption is reduced to a level lower than achieved in the prior art while maximizing the gain performance of the amplifier.
It is also an object of the invention to provide an amplifier in which quiescent current is set independently of the overall gain of the amplifier.
It is still a further object of the present invention to provide an amplifier which minimizes crossover distortion.